1. Field of the Invention
The present invention relates to a memory, and more particularly, it relates to a memory including ferroelectric capacitors.
2. Description of the Background Art
A ferroelectric memory is generally known as one of nonvolatile memories, as disclosed in Japanese Patent Laying-Open No. 2001-210795, for example. The ferroelectric memory utilizes pseudo capacitance change depending on the direction of polarization of a ferroelectric substance as a memory device. The ferroelectric memory is spotlighted as an ideal memory allowing data rewriting at a high speed and a low voltage in principle and having nonvolatility.
FIG. 23 is a circuit diagram showing the structure of an exemplary conventional 1T1C ferroelectric memory. Referring to FIG. 23, the exemplary conventional 1T1C ferroelectric memory comprises a plurality of memory cells 101, a reference voltage generation circuit 102 and a sense amplifier 103. The plurality of memory cells 101 are provided along a bit line pair BL100/BL101 consisting of bit lines BL100 and BL101. A plurality of word lines WL100 to WL103 and a plurality of plate lines PL100 and PL101 are provided to extend perpendicularly to the bit line pair BL100/BL101. The memory cells 101 are formed by single ferroelectric capacitors CF100 to CF103 holding data and single access transistors Tr100 to Tr103 consisting of n-channel transistors respectively. The ferroelectric capacitors CF100 to CF103 are constituted of first electrodes, second electrodes and ferroelectric films held between the first and second electrodes respectively. The first electrode of the ferroelectric capacitor CF100 (CF102) is connected to the plate line PL100 (PL101), while the second electrode thereof is connected to either the source or the drain of the access transistor Tr100 (Tr102). Either the drain or the source of the access transistor Tr100 (Tr102) is connected to the bit line BL100. The gate of the access transistor Tr100 (Tr102) is connected to the word line WL100 (WL102). The first electrode of the ferroelectric capacitor CF101 (CF103) is connected to the plate line PL100 (PL101), while the second electrode thereof is connected to either the source or the drain of the access transistor Tr101 (Tr103). Either the drain or the source of the access transistor Tr101 (Tr103) is connected to the bit line BL101. The gate of the access transistor Tr101 (Tr103) is connected to the word line WL101 (WL103).
The reference voltage generation circuit 102 is provided every bit line pair BL100/BL101. This reference voltage generation circuit 102 is so provided as to supply a reference potential Vref serving as a reference voltage for data determination to the sense amplifier 103 through the bit line BL100 or BL101. The reference voltage generation circuit 102 is formed by three n-channel transistors 104 to 106 and one capacitor 107. Either the source or the drain of the n-channel transistor 104 is connected to either the source or the drain of the n-channel transistor 105. Either the drain or the source of the n-channel transistor 104 is connected to the bit line BL100, while either the drain or the source of the n-channel transistor 105 is connected to the bit line BL101. Control signals DMP0 and DMPE for ON-OFF controlling the n-channel transistors 104 and 105 are input in the gates of the n-channel transistors 104 and 105 respectively. Either the source or the drain of the n-channel transistor 106 is connected to a node ND101 between either the source or the drain of the n-channel transistor 104 and either the source or the drain of the n-channel transistor 105. Either the drain or the source of the n-channel transistor 106 is supplied with the reference potential Vref. This reference potential Vref is generated by a reference potential generation circuit (not shown). A control signal DMPRS for ON-OFF controlling the n-channel transistor 106 is input in the gate of the n-channel transistor 106. A first electrode of the capacitor 107 is connected to a node ND102 between either the source or the drain of the n-channel transistor 104 and either the source or the drain of the n-channel transistor 105. A second electrode of the capacitor 107 is grounded.
The sense amplifier 103 is also provided every bit line pair BL100/BL101. This sense amplifier 103 is connected to the bit lines BL100 and BL101. A sense amplifier activation signal SAE is input in the sense amplifier 103 for activating the same. The sense amplifier 103 has a function of amplifying potential difference between the bit lines BL100 and BL101 by comparing a potential, corresponding to any of the data held in the ferroelectric capacitors CF100 to CF103, generated on either the bit line BL100 or the bit line BL101 with the reference potential Vref and determining the difference therebetween when the memory reads the data from the corresponding one of the ferroelectric capacitors CF100 to CF103. More specifically, the sense amplifier 103 is so formed as to set the potential generated on either the bit line BL100 or the bit line BL101 to the GND level when the potential, corresponding to any of the data held in the ferroelectric capacitors CF100 to CF103, generated on either the bit line BL100 or the bit line BL101 is lower than the reference potential Vref supplied from the reference voltage generation circuit 102 through either the bit line BL101 or the bit line BL100 in data reading. The sense amplifier 103 is also so formed as to amplify the potential generated on either the bit line BL100 or the BL101 to a level Vcc when the potential, corresponding to any of the data held in the ferroelectric capacitors CF100 to CF103, generated on either the bit line BL100 or the bit line BL101 is higher than the reference potential Vref supplied from the reference voltage generation circuit 102 through either the bit line BL101 or the bit line BL100 in data reading.
FIG. 24 is a voltage waveform diagram for illustrating operations of the exemplary conventional 1T1C ferroelectric memory. FIG. 25 is a hysteresis diagram showing a polarization state of the ferroelectric capacitor CF100 of the exemplary conventional 1T1C ferroelectric memory. The operations of the exemplary conventional 1T1C ferroelectric memory are now described with reference to FIGS. 23 to 25.
In an initial state of data reading, the potentials of the word line WL100, the plate line PL100 and the bit line BL100 are held at the GND level, as shown in FIG. 24. The potentials of the control signals DMPRS, DMP0 and DMPE and the sense amplifier activation signal SAE are also held at the GND level. In this state, the word line WL100 rises from the GND level to the potential Vcc. Thus, the access transistor Tr100 linked with the word line WL100 enters an ON-state. Further, the control signal DMPRS also rises from the GND level to the potential Vcc. Thus, the n-channel transistor 106 of the reference voltage generation circuit 102 also enters an ON-state. Therefore, the reference potential Vref is supplied through the ON-state n-channel transistor 106 for setting the nodes ND101 and ND102 to the reference potential Vref while charging the capacitor 107 with the reference potential Vref.
Then, the plate line PL100 rises from the GND level to the potential Vcc. Thus, the voltage Vcc is applied to the ferroelectric capacitor CF100 through the plate line PL100. Therefore, a read potential responsive to the data held in the ferroelectric capacitor CF100 is generated on the bit line BL100. If the ferroelectric capacitor CF100 holds data “0” at this time, the polarization state of the ferroelectric capacitor CF100 shifts from “0” to a point A along a hysteresis curve as shown in FIG. 25. Thus, the total charge quantity of the bit line BL100 linked with the ferroelectric capacitor CF100 is increased by a charge quantity Q0up shown in FIG. 25. Therefore, the potential of the bit line BL100 is increased in correspondence to the charge quantity Q0up. If the ferroelectric capacitor CF100 holds data “1”, on the other hand, the polarization state of the ferroelectric capacitor CF100 shifts from “1” to the point A along the hysteresis curve. Thus, the total charge quantity of the bit line BL100 linked with the ferroelectric capacitor CF100 is increased by a charge quantity Q1up shown in FIG. 25. Therefore, the potential of the bit line BL100 is increased in correspondence to the charge quantity Q1up. The increment Q1up of the total charge quantity of the bit line BL100 linked with the ferroelectric capacitor CF100 holding the data “1” is larger than the increment Q0up of the total charge quantity of the bit line BL100 linked with the ferroelectric capacitor CF100 holding the data “0”, as understood from the hysteresis diagram in FIG. 25. Thus, the potential of the bit line BL100 linked with the ferroelectric capacitor CF100 holding the data “1” is higher than the potential of the bit line BL100 linked with the ferroelectric capacitor CF100 holding the data “0”.
Then, the control signal DMPE rises from the GND level to the potential Vcc. Thus, the n-channel transistor 105 of the reference voltage generation circuit 102 enters an ON-state. Therefore, the reference potential Vref charged in the capacitor 107 is supplied to the bit line BL101 through the ON-state n-channel transistor 105. Thus, the potential of the bit line BL101 is held at the reference potential Vref. Then, the control signal DMPRS falls from the potential Vcc to the GND level. Thus, the n-channel transistor 106 of the reference voltage generation circuit 102 enters an OFF-state. Then, the control signal DMPE also falls from the potential Vcc to the GND level. Thus, the n-channel transistor 105 of the reference voltage generation circuit 102 enters an OFF-state. Therefore, the bit line BL101 enters a floating state (high-impedance state) while holding the reference potential Vref.
In this state, the sense amplifier activation signal SAE rises from the GND level to the potential Vcc, thereby activating the sense amplifier 103. Thus, the sense amplifier 103 determines and amplifies the data read from the ferroelectric capacitor CF100 by comparing the potential of the bit line BL100 corresponding to the read data and the reference potential Vref of the bit line BL101 with each other. In other words, the sense amplifier 103 determines the data read from the ferroelectric capacitor CF100 as “0” and sets the potential of the bit line BL100 to the GND level when the potential of the bit line is lower than the potential (reference potential Vref) of the bit line BL101. When the potential of the bit line BL100 is higher than the potential (reference potential Vref) of the bit line BL101, on the other hand, the sense amplifier 103 determines the data read from the ferroelectric capacitor CF100 as “1” and amplifies the potential of the bit line BL100 to the level Vcc. The potential of the bit line BL100 set to the GND level or the level Vcc by the sense amplifier 103 is output from the 1T1C ferroelectric memory.
In the exemplary conventional 1T1C ferroelectric memory shown in FIG. 23, the hysteresis of the ferroelectric capacitor CF100 may be dispersed due to dispersion of conditions for forming the ferroelectric film constituting the ferroelectric capacitor CF100. In this case, the potential (“0” potential) of the bit line BL100 linked with the ferroelectric capacitor CF100 from which the data “0” is read and the potential (“1” potential) of the bit line BL100 linked with the ferroelectric capacitor CF100 from which the data “1” is read are distributed as shown in FIG. 26 respectively. Depending on the hysteresis of the ferroelectric capacitor CF, further, the distribution range of the “0” potential may exceed that of the “1” potential as shown in FIG. 27, or the distribution range of the “1” potential may exceed that of the “0” potential as shown in FIG. 28. When the distribution ranges of the “0” and “1” potentials are dispersed as shown in FIGS. 26 to 28, it is disadvantageously difficult to set the reference potential Vref employed for determining the data “0” or “1”. Further, the distribution ranges of the “0” and “1” potentials may overlap with each other as shown in FIG. 29, depending on the hysteresis of the ferroelectric capacitor CF100. In this case, it is disadvantageously difficult to correctly determine the data, regardless of the level of the reference potential Vref.